JP2665912B2 - Insulation resistance measurement method that compensates for the effect of ground resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention relates to a method for measuring insulation resistance of an electric circuit or the like in a live state, and in particular, compensates for the influence of ground resistance which cannot be ignored when a stray capacitance to ground is large. To a simplified insulation resistance measuring method. (Prior Art) Conventionally, it has been general to measure the insulation resistance of an electric circuit by using a device as shown in FIG. That is, the second type ground line L _E of the power receiving transformer T, or pass through the transformer OT connected to the oscillator OSN for oscillating a low-frequency signal for measurements made different frequencies ₁ and the commercial power source frequency, or ground line the cut and the like for connecting the oscillator in series to apply a low-frequency voltage measurement paths L ₁ and L _2,
This After detecting a leakage current which returns to the ground line via an insulation resistor R _O and ground stray capacitance C _O exists between path and the earth by the ZCT that allowed through the ground line L _E Amplification is performed by the amplifier AMP, and only the frequency ₁ component obtained by removing the commercial frequency component by the filter FIL is selected, and this is applied to the rectifier DET to measure the insulation resistance of the electric circuit using the voltage obtained. This circuit can be represented by an equivalent circuit shown in FIG. In the same Figure 4, R _O is the measured circuit insulation resistance, C _O is also ground stray a capacity of measuring the low-frequency oscillator OSC which flows induced in the ground line L _E in the circuit under measurement output The signal is R
_It shows a case where the signal returns to the ground line again via _O and _CO . Note that r is a ground resistance between the ground point E and the ground. Conventionally, the insulation resistance has been determined from the following calculation based on such an equivalent circuit. That is, in the same figure, the current flowing through the oscillator OSC having the frequency ₁ via the ground point E is _1, and this is ₁ = (A + jB) (1). At this time (However, ω ₁ = 2π ₁ ). Generally, R _O ≫r, and if ω ₁ is selected so that (ω ₁ C _Or ) ² ≪1 (4), the above equation (2) becomes Since the above equation (3) can be expressed as Bω ₁ C _O (6), the above-mentioned A and B can be obtained by actually measuring the feedback current ₁ , and the insulation resistance R _O can be obtained therefrom. Can be. However, according to the conventional insulation resistance measuring method as described above, when the stray capacitance C _O to the ground is large, and when the frequency ω ₁ / 2π of the applied voltage is high, the stray capacitance C _In addition to the fact that an accurate value of the insulation resistance R _O cannot be obtained because the component involving _O increases, the effect of the ground resistance r cannot be neglected if the value is further increased, making the measurement itself impossible. . (Object of the Invention) The present invention has been made to eliminate defects in such a conventional method for measuring insulation resistance of an electric circuit, and provides an insulation resistance measurement method which compensates for the influence of ground resistance. It is. (Summary of the Invention) In order to achieve the above object, an insulation resistance measuring method according to the present invention comprises a transformer core having a low-frequency signal applied to a ground wire or an electric circuit of a transformer and a current transformer for detecting a leakage current. And a new connecting line is provided through the transformer core and the current transformer to apply a signal of the opposite phase of the low-frequency signal applied to the core of the transformer to the current transformer. Terminate with a variable capacitor. First, in this state, the ground capacitance C _O is detected by adjusting the first variable capacitor so that the low-frequency leakage current component included in the current transformer output is minimized.
Determining the magnitude I ₁ of the low-frequency component obtained after the adjustment.
Next, a capacitor having a predetermined value C is forcibly inserted between the electric circuit and the ground, and the first capacitor is adjusted so that the low-frequency leakage current component contained in the output of the current transformer is minimized. The magnitude I ₂ of the low frequency component obtained by the above is obtained, and using these two components I ₁ and I ₂ (here The following calculation is performed to measure the insulation resistance of the electric circuit. Hereinafter, the present invention will be described in detail based on illustrated embodiments. One embodiment of the present invention is shown in FIG. The same symbols as those in FIG. 3 have the same meaning. In FIG. 1, the transformers OT and the current transformer ZCT pass through the electric circuits L ₁ and L _2. The operation of applying a low-frequency voltage to the electric circuit and detecting the leakage current is shown in FIG. They are completely equivalent. This embodiment differs from FIG. 3 in that the connection line Lp is further passed through the transformer OT and the current transformer ZCT which penetrate the electric lines L ₁ and L ₂ so that the phase is opposite to that of the electric line. and it was terminated at a point of connecting the second capacitor C through a switch SW between the ground path L ₂ and ground. The operation of this device is as follows. Frequency ₁ = ω ₁ /
By applying the output of an oscillator oscillating a low frequency of 2π to the transformer OT, a voltage of V _sin ω _1t is applied between the electric circuit and the ground. First, considering that there is no connection line Lp, the current transformer ZCT
Is amplified by the amplifier AMP and applied to the filter FIL which detects the component of the frequency ₁ , the output ₁ of the filter is obtained from the equations (1), (5) and (6). Is represented by Next, if the connection line Lp, which passes through the current transformer ZCT and the transformer OT in a phase opposite to that of the electric circuit, is terminated with a capacitor Cv, the current flowing through the connection line Lp is -jω ₁ Cv, so the filter at this time The output ₁ 'of FIL is Becomes Therefore, if the filter output ₁ 'is applied to the rectifier DET, then the rectifier output will be ... (9) Therefore, by adjusting the variable capacitor Cv such that the rectifier output is minimized, can be a minimum becomes the earth capacitance of the electrical path when the C _O = Cv measured from the value of the capacitor Cv, the C _O = Cv if the rectifier output when the I ₁ Becomes Next, a capacitor C having a predetermined value is inserted between the measurement circuit and the ground. This can be done, for example, by connecting the capacitor C to the ground via the switch SW to the ground phase (here, L ₂ ) of the electric circuit as shown in FIG. 3, and when the capacitor C is inserted (when the switch SW is “ON”). ) Is the filter FIL output ₂ ' Since the, by applying a filter output to the rectifier DET, if rectifier output, by adjusting the variable capacitor Cv to minimize becomes C _O + C = minimum time of Cv, if the rectifier output at this time is I ₂ ( 11) From equation Becomes At this time, (C _O + C = Cv) (10) Equation (12) is If we take the ratio of equations (13) and (14), Here, If formula (15) is further modified, Becomes That is, k shown in the equation (16) is calculated from the measured value of the ground capacitance C _O and the value of the capacitor C, and the measured values I ₁ and I ₂ are substituted into the equation (17). Can be measured. In the above example, a single-phase two-wire circuit is shown as an example. However, other single-phase three-wire and three-phase three-wire circuits can be similarly measured. Further, in the above example, the connection line is simply passed through the transformer OT and the current transformer ZCT. However, if the connection line is wound N times only on the transformer OT, the current flowing through the connection line becomes −jω ₁ CvN, This is equivalent to making the value of the capacitor Cv 1 / N. The same is true even if winding is performed N times only on the current transformer ZCT. Further, if winding is performed on both of them a predetermined number of times, it is possible to reduce the value of the capacitor Cv by the respective products. In the above description, the capacitor Cv is used to pass a current shifted by 90 degrees from the applied low-frequency voltage to the connection line Lp. However, in order to pass such a current, the capacitor Cv is passed through or wound around the transformer OT as shown in FIG. By applying the wire Lq to the 90-degree phase shifter ps, a voltage 90 degrees shifted from the applied voltage is generated.
The output is applied to the power amplifier PAMP, the output of which is passed or wound through the current transformer, the conductor Lr is terminated with a resistor Rv, and the current flowing through the conductor Lq is opposite to the current flowing due to the earth capacitance flowing through the electric circuit. Obviously, by adjusting the value of the resistor Rv, it is possible to realize a function equivalent to adjusting the capacitor Cv in FIG. Alternatively, the value of the resistor Rv may be fixed, and the output voltage of the power amplifier may be adjusted to be variable. (Effects of the Invention) As described above, the method of the present invention can compensate for the influence of the ground resistance, so that the frequency of the applied voltage can be increased as compared with the conventional method, and the size of the transformer OT can be reduced.
This is effective for realizing a small measuring instrument.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing another embodiment of the present invention, FIG.
FIG. 4 is a diagram showing an equivalent circuit of an insulation resistance measuring system of an electric circuit. T: Power receiving transformer, ZCT: Current transformer, OT: Transformer, AM
P …… Amplifier, FIL …… Filter, DET …… Rectifier, OSC ……
Oscillator, SW ... Switch.

Claims (1)

(57) [Claims] A transformer core for applying a low-frequency signal to a ground wire or an electric circuit of a transformer and a current transformer for detecting a leakage current are coupled, and a signal having a phase opposite to that of the low-frequency signal applied to the transformer core is formed. Providing a new connection through the transformer core and the current transformer to apply to the current transformer;
A first capacitor is inserted into the connection line, and a second capacitor having a predetermined value is provided between the electric circuit and the ground via switching means, and the second capacitor is inserted between the electric circuit and the earth. The value of the first capacitor is adjusted so that the leakage current component of the low-frequency signal included in the output of the current transformer is minimized in a state where the current is not transmitted, and the leakage current component of the low-frequency signal obtained by this adjustment is adjusted And the size of the first capacitor so that the leakage current component of the low-frequency signal included in the current transformer output is minimized in a state where the second capacitor is inserted between the electric circuit and the ground. A ground resistance, wherein a value of the leakage current component of the low-frequency signal obtained by the adjustment and a value of the first and second capacitors are used to calculate an insulation resistance of the electric circuit. Absolutely compensated for the effects of Resistance measurement method. 2. 2. The method according to claim 1, wherein the connection line is wound around one or both of the transformer core and the current transformer.